Separate metadata step for NL target

[kad-korpem]

Description

The tags for the NL target weren't being set correctly, with the -nl suffix only being applied to the last of the tags:

pdok/mapserver:{major}
pdok/mapserver:{major}.{minor}
pdok/mapserver:{major}.{minor}.{patch}-nl

This meant that the tags for {major} and {major}.{minor} would be overwritten, and thus only exist for the NL target.
By adding a separate docker/metadata-action for the NL target, the tags should now all have the suffix:

pdok/mapserver:{major}-nl
pdok/mapserver:{major}.{minor}-nl
pdok/mapserver:{major}.{minor}.{patch}-nl

Type of change

• Improvement of existing feature
• Bugfix
• Configuration change

Checklist:

• I've double-checked the code in this PR myself
• I've left the code better than before (boy scout rule)
• The code is readable, comments are added that explain hard or non-obvious parts.
• I've expanded/improved the (unit) tests, when applicable
• I've run (unit) tests that prove my solution works
• There's no sensitive information like credentials in my PR